1. Field of the Invention
The invention relates to a liquid droplet ejecting apparatus including a temporary storing chamber which stores a liquid as supplied from a main tank, and from which the liquid is supplied to an ejecting head having nozzles, from which the liquid is ejected in the form of droplets. In particularly, the invention relates to a liquid droplet ejecting apparatus having a purging mechanism for restoring an ejection performance of the apparatus.
2. Description of Related Art
In an inkjet recording apparatus as a kind of a liquid droplet ejecting apparatus, as the recording apparatus is used for a relatively long term, a liquid may evaporate from an ink, with which an image is formed by ejecting droplets thereof onto a recording medium from nozzles formed in a recording head in the recording apparatus, thereby making the ink highly viscous, and also bubbles of air may be accumulated in the recording head. The air bubbles in the recording head are produced such that the air is introduced from the exterior to the interior of the recording head through nozzles, or the air dissolved in the ink aggregate and grow into the bubbles in the ink as temporarily stored in a sub tank as a temporary storing chamber. Further, the air bubbles may be produced when the ink in the sub tank is shook or moved during the inkjet recording apparatus is transferred. The highly viscous ink and the bubbles in the recording head clog the nozzles to deteriorate the ink ejection performance of the recording head, namely, droplets of the ink may not be ejected as desired in forming an image. In addition, the ink may contain a foreign material such as dust and dirt and ink powder or the ink as dried. Such a foreign material may also cause the clogging of the nozzles. Since the thus deteriorated ink ejection performance leads to degradation in the quality of the formed image, an inkjet recording apparatus typically includes a purging mechanism for implementing a restoring operation for eliminating the highly viscous ink, bubbles, and others, in order to restore the ink ejection performance of the recording head to the initial, excellent level.
The restoring operation is roughly divided into a purging operation, which may be implemented cyclically for instance, and a flashing operation (or a preliminary ejection operation). The purging operation is implemented to discharge the fresh ink along with the highly viscous ink, the ink powder, the air bubbles, the dust, and the others, by (i) forcibly sucking the fresh ink from the exterior of the recording head through the nozzles, by applying a negative pressure, or (ii) forcibly pressure-feeding the fresh ink into the recording head from the upstream side of the recording head with respect to an ink communication passage that communicates a main tank with the nozzles in the recording apparatus. On the other hand, the flashing operation is implemented mainly during a continuous operation of the recording apparatus, in order to discharge the ink whose viscosity has been increased during the operation, from the nozzles. It is usual that the flashing operation is implemented more frequently than the purging operation, but the amount of the ink as discharged along with the viscous ink and others in the flashing operation is smaller than that in the purging operation.
In the former way of purging where the ink is sucked by a negative pressure to eliminate the bubbles and the foreign material, a cap is used to cover the nozzles so as to receive the ink as discharged or sucked from the recording head. Hence, the fresh ink is necessarily sucked to an amount corresponding to an inner volume of the cap, leading to a large volume of the ink wasted.
In the latter way of purging (i.e., “the positive-pressure purging”) where the ink is applied with a positive pressure from an internal side of the recording head opposite to the nozzles (that is, from a side from which the ink is supplied) so that the ink is pressure-fed into the recording head, the amount of the ink wasted can be reduced as compared with the former way of purging. That is, in the latter way also, the fresh ink is wasted since the ink is discharged along with the bubbles and foreign material, but by applying the positive pressure for a short time period and quickly eliminating the positive pressure, the amount of the waste ink can be made relatively small.
JP-A-10-151761 (hereinafter referred to as “first publication”) discloses an inkjet recording apparatus including a pump used in the purging operation. The pump is operated to pressurize an ink tank storing an ink, in order to eliminate bubbles and a solid material in an inset recording head and an ink communication passage in the recording apparatus.
However, the inkjet recording apparatus disclosed in the first publication has a drawback that even when the pump is reduced in size, employment of the pump and a driving device for the pump is essential, making the structure of the apparatus complex, and an overall size of the apparatus large.
JP-A-11-235831 (hereinafter referred to as “second publication”) discloses an inkjet recording apparatus including an ink cartridge that stores an ink and has a lid. The lid has a protrusion that pressurizes the ink in the ink cartridge as the lid is closed, in order to forcibly pressure-feed the ink to an inkjet recording head, thereby accomplishing the purging operation.
However, the inkjet recording apparatus disclosed in the second publication where the lid is manually closed by a user to apply a pressure to the air and in turn the ink in the ink cartridge, has a drawback that a speed at which the lid is closed fluctuates, thereby causing a variation in the applied pressure, and making it difficult to stably implement the purging operation.
It is also known to use, in the positive-pressure purging, a valve to eliminate the once applied positive pressure, namely, the valve is opened when the positive pressure is to be eliminated. In operation, the opening of the valve should be made instantly and stably in order to control the amount of the waste ink. Thus, it is desirable to use a solenoid valve having a stable opening characteristic. However, a solenoid valve is generally large in size and high in price, which works against the downsizing and cost reduction of the inkjet recording apparatus. Thus, in practice, a solenoid valve can not be used in view of the cost effectiveness.
To solve this problem, Japanese Patent No. 2819639 (hereinafter referred to as “third publication”) for instance, discloses a pump mechanism including a plunger and a cylinder for pressurizing an ink stored in a sub tank to pressure-feed the ink into a recording head, in order to restore an ink ejection performance of the recording head.
In this pump mechanism for restoring the ink ejection performance, the plunger is slidably received in the cylinder that is vertically displaceable in the sub tank. The plunger is normally biased by a restoring spring to an upper position to open an ink supply port formed at a bottom of the ink tank. An O-ring is interposed between the cylinder and the plunger to seal therebetween.
When a head portion of the plunger is pushed downward against a biasing force of the restoring spring, the cylinder is initially displaced to a position to cover the ink supply port and stops there. Then, by the head portion of the plunger being further pushed down, the plunger slides down in the currently stationary cylinder. Since a bottom of the cylinder covering the ink supply port has an opening, the plunger, as pushed down to the position to contact the bottom of the ink tank, pressure-feeds the ink into the recording head, thereby discharging the ink containing the bubbles and others to the exterior through the nozzles.
Upon termination of the pushing of the plunger, the plunger and the cylinder are integrally displaced upward by the biasing force of the restoring spring. Then, the cylinder is brought into contact with an inner wall surface of the ink tank and stops there, and thereafter only the plunger is displaced to its original position by the spring force of the restoring spring.
The pump mechanism for restoring the ink ejection performance is disadvantage in the following. The O-ring disposed between the cylinder and the plunger to allow the relative sliding movement in friction between the plunger and the cylinder may be damaged by wear or secular change or for other reasons. This causes the cylinder to fall to the lowermost position in the ink tank by its, own weight, and then the cylinder is held there. This means that the bottom of the cylinder keeps closing the ink supply port, inhibiting the supply of the ink, whether by the pressure-feeding or not, into the recording head.
Further, since the technique of the third publication requires the O-ring and the vertically displaceable cylinder, the number of components and accordingly the cost of the recording apparatus are large, and also the number of production steps can not be reduced.
Meanwhile, JP-A-5-92578 (hereinafter referred to as “fourth publication”) (see FIG. 1) for instance, discloses, as another example of the positive-pressure purging, an arrangement where an air pressure pump is used to compress the air in a sub tank, in turn applying a positive pressure to the ink in the sub tank so as to pressure-feed the ink to the downstream side with respect to an ink communication passage into a recording head, from which the ink is discharged to the exterior.
However, in the arrangement of the fourth publication, the pressurizing of the ink is made such that initially the air is compressed to produce a pressure which is then transmitted to a surface of the ink in the sub tank. Thus, the pressure loss is large, making the purging operation inefficient. In addition, by the compression of the air, the ink may flow in the reverse direction also, that is, the ink may flow back toward an ink supply source as well as toward the recording head, thereby further lowering the efficiency of the purging. In order to prevent the ink flow in the reverse direction, it is essential to dispose a check valve at an appropriate position.
To restore the ink ejection performance of the recording head, it is necessary to discharge the ink from the nozzles in an amount sufficiently large in each discharging, or at a speed sufficiently high. In the arrangement where the air is compressed first, the speed of the ink flow rises relatively slowly, due to the large pressure loss. Hence, before the speed reaches the sufficient level for purging, a large amount of the ink flows out of the recording head, leading to much waste of ink.
Further, in the arrangement of the fourth publication where the air pressure pump is used to compress the air in the sub tank in order to restore the ink ejection performance of the recording head, the air pressure pump is required in the purging mechanism, and a drive source and a link mechanism for transmitting a driving force for the air pressure pump are also required. Hence, an overall size of the apparatus and the cost are increased, while the load imposed on the purging mechanism is high.
To overcome the drawbacks of such an arrangement involving the air compression, JP-A-7-232436 (hereinafter referred to as “fifth publication”) for instance, discloses another arrangement for the positive-pressure purging, which includes a head case, a sub tank (or an ink sack) of elastic material which is accommodated in the head case and storing an ink, and a pressure chamber at least a part of which is formed of an elastic member, and which is in communication with the head case. In the purging operation, in order to discharge the ink from the nozzles, a user manually or with fingers presses the elastic member to change an inner volume of the head case, thereby applying a positive pressure to the sub tank to reduce an inner volume of the sub tank.
In the arrangement of the fifth publication where the ink discharge is made by a change in the inner volume of the head case, the speed of the ink flow rapidly rises up to the level sufficient for the purging operation, thereby reducing the amount of the ink wasted in the purging operation. However, this arrangement may generate ink flow in the reverse direction toward the ink supply source when the pressure applied to change the inner volume is eliminated, thereby lowering the efficiency. Hence, in this arrangement, too, a check valve is essentially disposed.
Further, when the inner volume of the head case is restored, or when the sub tank is restored to its original shape, after the purging operation, a negative pressure is produced at the nozzles, thereby causing flow of the ink in the reverse direction from the nozzles back into the recording head. Depending on the magnitude of the negative pressure, meniscuses formed in the nozzles may be broken.
The arrangement of the fifth publication, where the elastic member is pressed to pressurize the air in the head case and in turn presses the sub tank in order to purge the nozzles, can omit the air pressure pump and the associated devices, but requires the user to manually press the elastic member each time the purging operation is to be implemented. This troubles the user very much, deteriorating the user-friendliness.
All the above-described drawbacks are seen not only in the inkjet recording apparatus, but also in various kinds of liquid droplet ejecting apparatuses where a liquid stored in a sub tank is supplied to an ejecting head having a nozzle from which the liquid is ejected in the form of droplets.